Patterns may be fabricated on a semiconductor (e.g., a silicon wafer) by transmitting beams of light through a mask and onto a surface of the semiconductor. To produce patterns with extremely small pitches (i.e., the distances between lines or features), a series of resolution enhancement techniques (RETs) have been employed to enhance a resolution limit of optical lithography while providing a manufacturable depth of focus (DOF). A principle RET applied in low k1 lithography in the fabrication of semiconductor devices is off-axis illumination (OAI), which has been shown to be effective in increasing DOF while improving image resolution. While OAI may be effective for a narrow range of applications, for example a pattern layout with a densely packed series of repeated features, the process window for layouts of features combining regions of isolated and dense patterns may be vanishingly small.
One method for enhancing the lithography process window is to employ an illumination aperture in an illuminator assembly of a projector system. Referring now to FIG. 1 (Prior Art), the basic components that make up a projection system for photolithography are schematically illustrated. A light beam 105 is condensed by illuminator lens 110 so that reticle 115 that includes feature 120, is uniformly illuminated. Most of the light beam 105 passes straight on as the zero order diffraction maximum 125, while first order diffraction maxima 130 and higher order diffraction maxima 135 are diffracted off to the side. These are then focused by projection lens 140 onto focal plane 145. Since no information (other than overall brightness) is contained in the zero order diffraction maximum 125, it is imperative that at least some of the higher order beams, such as the first order diffraction maxima 130 and higher order diffraction maxima 135, contribute to the image. This necessarily widens the angle of the focusing cone, resulting in a reduced DOF.
In FIG. 2 (Prior Art), the basic setup of FIG. 1 has been modified so that light beam 105 is blocked from the center of illuminator lens 110 by an illumination aperture filter 210, being limited to coming in obliquely (off-axis). The result of this is that the zero order diffraction maximum 125 is forced over the to the edge of projection lens 140 while first order diffraction maxima 130 passes (approximately) through the center of the projection lens 140, thereby allowing a narrower angle for the focusing cone, with a corresponding increase in DOF.
In FIG. 3 (Prior Art), a different modification of the basic setup of FIG. 1 has been introduced. This is the placement of phase-type filter 310 at a pupil plane of the projection lens 140. Its effect is to change the phase of the first order diffraction maxima 130 and higher order diffraction maxima 135 by 180 degrees relative to that of the zero order diffraction maxima 125. This results in an increase of DOF for dense patterns.